Bandgap references are high-performance analog circuits that are applied to analog, digital and mixed-signal integrated systems. For such applications, the accuracy of the bandgap reference voltage is a significant component of system functionality, important particularly in such precision applications as converters. Bandgap references use the bandgap voltage of underlying semiconductor material (often crystalline silicon) to generate an internal DC reference voltage that is based on the bandgap voltage.
Many bandgap references forward bias the base-emitter region of a bipolar transistor to form a voltage VBE across its base-emitter region. VBE is then used to generate the internal DC reference voltage. VBE, however, exhibits some first-order, second-order and higher order temperature dependencies. Many bandgap references substantially eliminate the first-order temperature dependency by adding a Proportional-To-Absolute-Temperature (PTAT) voltage to VBE.
One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 3,887,863 to A. P. Brokaw. The bandgap voltage reference circuit disclosed in the '863 patent relies upon a bandgap cell that is commonly referred to as a “Brokaw cell.” Referring to FIG. 1 of the drawings herein, Brokaw cell 100 comprises a pair of bipolar transistors (Q1 and Q2) and a pair of resistors (R1 and R2). The area of the base-emitter regions in Q1 and Q2 are indicated by A and unity, respectively, wherein A is greater than unity.
A bandgap voltage reference circuit 200 incorporating a Brokaw cell 100 is shown in FIG. 2. In addition to the Brokaw cell 100, the bandgap voltage reference circuit 200 comprises an operational transresistance amplifier R, as well as a pair of resistors R3 and R4 that allow the reference output voltage (VOUT) to exceed the bandgap voltage.
During operation, a voltage of VBE develops across the base-emitter region of bipolar transistor Q2. In addition, a PTAT voltage (termed VPTAT) develops across resistor R2. The base-emitter voltage (VBE) of a bipolar junction transistor has a negative temperature coefficient generally between −1.7 mV/degree C. and −2 mV/degree C. In contrast, the PTAT voltage has a positive temperature coefficient. By matching the temperature coefficient of VBE of Q2 to the temperature coefficient of VPTAT) of R2, the first order temperature coefficient of VBE can be made to be nearly zero, thereby significantly reducing temperature dependency.
Although the described bandgap voltage reference circuit substantially eliminates first-order temperature dependencies in the output voltage, second and higher order temperature dependencies tend to persist. A plot of output voltage as a function of temperature yields an approximately parabolic curve that reaches a maximum at about the ambient temperature of the bandgap reference.
Some bandgap references have reduced second and higher order temperature variations in the output voltage. One such bandgap voltage reference circuit is disclosed in U.S. Pat. No. 5,767,664 to B. L. Price. FIG. 3 of the drawings herein illustrates such a bandgap reference 300, which is shown to include the conventional bandgap reference 200 of FIG. 2, as well as a V-to-I converter circuit 304 with two differential pair segments 306 which are made up of MOSFETs M1-M4. A current mirror 308 is formed with MOSFETs M5 and M6 so as to extract a correction current, ICORR, from the VB node. The correction current reduces a significant portion of the remaining temperature dependencies present in the bandgap reference 200. Accordingly, the voltage at node VB is relatively temperature stable, and as a consequence, the output voltage of the bandgap reference 300 is a DC voltage that similarly is relatively stable with temperature changes compared to uncompensated bandgap reference 200.
Although effective for the purpose intended, the '664 bandgap reference curvature correction circuit has disadvantages. For example, in the '664 circuit, the correction current supplied to the reference requires some bandgap multiple as an output, that is, the bandgap requires gain. In addition, as the correction current is developed across a feedback resistor, that resistor must match the bandgap core resistors. The feedback resistor also will have to match the output voltage divider string to precisely set the gain. Thus, all the resistors need critical matching to each other. Furthermore, the '664 circuit implements a current mirror circuit to source compensation current, that will tend to impose magnitude and drift error. The inventive subject matter described herein addresses these and other concerns.